Filters are utilized in RF electronic circuits in many important technological applications. In high volume applications such as wireless mobile telecommunications filter technologies today are dominated by Surface Acoustic Wave (SAW) and microwave ceramics. Ceramic filters are both high performance and low cost but they suffer from having a large footprint on a wireless board. SAW filters exhibit size advantages over ceramic filters but suffer from a relatively poor sensitivity to temperature and a limited power handling capability.
The need to reduce cost and size in wireless equipment has led to a continuing need for even smaller and lower cost filters. One class of filter components that has the potential to meet these needs is based on Bulk Acoustic Wave (BAW) devices. Like SAW devices, BAWs use the piezoelectric effect to convert electrical energy into mechanical energy resulting from an applied RF voltage. Unlike SAWs, however, the energy is directed into the bulk. BAW devices have a superior power handling capability compared to SAWs, are less sensitive to surface contamination, and have the potential for a very small footprint.
BAW devices generally operate at their mechanical resonant frequency which is defined as that frequency for which the half wavelength of sound waves propagating in the device is equal to the total device thickness for a given velocity of sound for the material. BAW resonators operating in the GHz, range generally have physical dimensions of tens of microns in diameter with thicknesses of a few microns.
In order to ensure proper functionality, the piezoelectric layer of the BAW device needs to be acoustically isolated from the substrate. In the prior art there are two methods that have been used to achieve this acoustic isolation. The first of these is illustrated in prior art FIG. 1, which shows a typical Thin Film Bulk Acoustic Resonator (FBAR) structure. This type of device is discussed in detail by Ruby et al. (U.S. Pat. No. 6,060,818, issued May 9, 2000). In the FBAR device the acoustic isolation of the piezoelectric layer is achieved by removing the substrate 100 or an appropriate sacrificial layer from beneath the resonating component. The other components of the FBAR device consist of an air gap cavity 110, an insulating oxide layer 120, the piezoelectric layer 130 and the drive electrodes 140. Although FBARs have excellent energy confinement because of the air gap, they are prone to spurious modes being superimposed on the fundamental resonance mode. This is likely because of the free nature of the FBAR resonator and the large number of degrees of freedom present that can support a large number of modes. In addition, FBARs are difficult to manufacture particularly with regard to the packaging of such a structure. Generally a high vacuum is required within a packaged FBAR to allow a high quality factor or Q in the finished device.
The second method of providing acoustic isolation is shown in prior art FIG. 2 which illustrates a Solidly Mounted Resonator (SMR). This type of device is described in detail by Aigner et al. (U.S. Pat. No. 6,841,922, issued Jan. 11, 2005). In terms of robustness, the SMR is superior to the FBAR, given that there is little risk of mechanical damage in any of the standard processes used in dicing, assembly and packaging.
In an SMR device the acoustic isolation is achieved by growing the resonator on top of a highly efficient acoustic reflector. The piezoelectric resonator 160 comprises a piezoelectric layer 170 as well as a set of electrodes 180. An acoustic Bragg reflector 190 is located between the resonator 160 and the substrate 200. The acoustic Bragg reflector consists of a plurality of layers 191 to 197. Layers 191,193,195 and 197 of the acoustic reflector are layers with high acoustic impedance and layers 192, 194 and 196 are layers with low acoustic impedance. The thickness of each of these layers is fixed to be one quarter wavelength of the resonant frequency. The greater the number of alternating layers present in the acoustic Bragg reflector 190 the greater is the efficiency of the reflector. The efficiency of the acoustic Bragg reflector is also dependant on the mismatch between the acoustic impedances. The greater the difference in acoustic impedance between the low and high acoustic impedance materials, the more efficient the reflector. If the materials used in the reflector have only a small difference in acoustic impedance then more alternating layers will be required to achieve the same performance, leading to higher device complexity.
Acoustic impedance is the acoustic analogue of a material's optical index of refraction. Materials with high acoustic impedance are metals such as tungsten (W), platinum (Pt), molybdenum (Mo) or gold (Au). Examples of materials with low acoustic impedance are silicon oxide (SiO2) and aluminum (Al). W/SiO2 layers have been used in prior art devices to produce efficient reflectors. However, there are drawbacks to incorporating a metal into the acoustic Bragg reflector. Since metals such as W are conductive, and in order to avoid parasitic coupling between neighboring resonators on a filter chip, the reflector needs to be patterned. Also, if silicon is used as a substrate, this can induce parasitic capacitive coupling between the resonator bottom electrodes and the substrate through the metal reflector layers. Such coupling degrades the resonator's electrical performance.
These drawbacks of using a metal as one of the components in an acoustic Bragg reflector are well recognized in the art and attempts have been made to use an all-dielectric reflector that would negate the problem associated with parasitic capacitances. For example, Naik et al. (IEEE Trans., Ultrasonics, Ferroelectrics, and Frequency Control, 47(1), 2000, pp. 292-296) employed an all-dielectric reflector consisting of alternating layers of aluminum nitride (Al—N) and SiO2. However, due to the small difference in acoustic impedance between these two materials only a comparatively inefficient reflector can be constructed for the same number of alternating layers used in a metal/SiO2 stack such as W/SiO2.
Thus there is a need to provide a SMR structure which has less problems associated with packaging than an FBAR but which does not suffer from the parasitic capacitance problems that are present in SMRs that utilize metal layers in the acoustic Bragg reflector. There is a further need to provide an SMR structure that uses an all-dielectric acoustic Bragg reflector but at the same time has as high an efficiency for the same number of layers as a metal/SiO2 acoustic Bragg reflector such as a W/SiO2 reflector.